Oil refineries, petrochemical plants, and other facilities that process hydrocarbons run equipment 24 hours a day, seven days a week to maximize efficiency. The goal of these facilities is to remain operational for as long as possible and to avoid downtime whenever feasible. However, regardless of continuous run times and steps taken to continue operations, all facilities must, at some point, stop operations to take the process equipment out of service for repairs, upgrades, and inspections. This activity is often called a “turnaround” and is a major event for the unit. The facilities experience significant financial loss for every hour that the equipment is out of service, so getting the equipment ready for maintenance as quickly as possible is desired.
During a turnaround, the equipment cannot merely be shut down and drained before performance of maintenance and inspections. Rather, several steps must be taken to decontaminate the equipment before the facility personnel may open and expose the equipment to the atmosphere. The facility must first stop the feed from entering the equipment and remove any material that is present within the internal structure. While the equipment may seem empty at this point and may be under a steam or nitrogen atmosphere, the system cannot be exposed to the air, due to the noxious qualities of the vapors remaining in the equipment. Indeed, before opening the equipment and exposing personnel to the noxious vapors, the facility must ensure the amount of each contaminant, for example, hydrogen sulfide (H2S) and benzene, is within acceptable ranges set forth by government and facility regulations. The amount of each contaminant must be within a safe, acceptable level prior to opening the equipment to prevent any risk of fire, harmful exposure to personnel, or environmental damage. The contaminants are typically checked in the vapor space with meters or lab equipment and the equipment remains closed to the atmosphere until satisfactory readings are achieved. The process of lowering the amount of contaminants in the vapors to an acceptable level for the equipment to be exposed to the atmosphere and ready for personnel entry is called decontamination or cleaning.
Over the years, several methods have been developed to decontaminate or clean the equipment before exposing the equipment to the atmosphere or facility personnel. The most common decontamination process includes the use of steam, nitrogen, or air to strip out the contaminants. Many process vessels today have a hard piped steam connection that was used for this “steam-out” process. Over an extended period of time, any of these gas streams will lower the noxious vapor content of the equipment, but there are numerous limitations to this method. For instance, this “steam-out” process requires a substantial amount of time to initially clear noxious gases, such as hydrogen sulfide (H2S), and benzene. Steaming a vessel could take more than a week to complete. Another downside of this process is the possibility of noxious vapors reappearing if personnel disturb any material in the equipment. Indeed, contaminants, such as benzene, can be trapped beneath scale or other waste product and seep out at later time (e.g., when cleaning had been considered completed).
An improved method for decontamination was developed that involves circulating an aqueous solution of a chemical cleaner, sometimes while also applying steam or heat. The makeup of the aqueous chemical cleaners is varied and includes such components as solvents, surfactants, scavengers, enzymes, acids, and caustics. Although each chemical blend can be quite different, the general method of applying these aqueous chemicals is similar. The process usually involves circulating the cleaning solution with pumps and testing the solution for when it is spent. During circulation, the gas content of the vessel is usually checked and the chemical cleaning is normally stopped when the contaminant vapor content reaches the desired ranges. This type of aqueous chemical cleaning results in cleaner equipment than steam or nitrogen purging with less reappearance of noxious gases. However, as with the previous method discussed above, there are deficiencies in this process. First, the volume of effluent waste generated can be significant and difficult to treat. Second, the time to completely treat a vessel remains long, often lasting several days. Third, the equipment is only cleaned where it is contacted with the liquid chemical cleaner. Much of a larger vessel may remain untreated due to poor distribution of the chemical cleaner, especially when liquid channeling occurs and with the bottom sides of equipment internals. Finally, the aqueous chemical solutions have a limited ability to dissolve or remove heavier hydrocarbon materials that are inside the equipment. This material may give off noxious fumes or hamper planned mechanical work or visual inspections.
Another cleaning process has also been used in the industry that utilizes terpene solvents. According to this process, terpene solvents are injected into equipment with steam and an additive package. The steps of this process are similar to the “steam-out” process, but include the additional step of injecting the terpene based chemical into the steam going into the equipment. In this process, no liquid circulation is required so equipment requirements are lower and the volume of effluent liquid generated is significantly less. This process is also faster than the previous methods discussed above, often decreasing the decontamination process by more than one day. However, the terpene solvent steam cleaning approach is very costly. While terpenes may have beneficial physical properties for use in the process, the raw material costs for terpenes are very high and are much more expensive than other cleaning solutions. In addition, because terpenes are not derived from crude oil, it is necessary for the refineries and plants to closely track the use of the terpene during the cleaning process. In particular, terpene is a volatile unsaturated hydrocarbon found in the essential oils of plants, especially conifers and citrus trees, which means that it has one or more unsaturated double bonds between the carbon atoms. Accordingly, since terpenes and other unsaturated hydrocarbons have high reactivity and a tendency to polymerize, unsaturated compounds generally would not be expected to be present in large quantities in crude petroleum. Thus, constant tracking of the terpene is necessary, but can be burdensome and time-consuming.
All of the above-mentioned processes have associated drawbacks and limitations that significantly hinder the speed and efficiency of the decontamination process during a turnaround. Accordingly, there remains a need for an improved process for the cleaning and decontamination of equipment during a turnaround that is faster, more efficient, and can be performed at a significantly lower cost.